Filters

Filters
allow you to set rules for distributing incoming emails to different mailboxes.
Typically this is achieved by setting rules for header fields. Let's
say you use Eudora for two different email addresses. You can set filters
so all of your "mail.utexas.edu" emails go into one mailbox and your
"ischool.utexas.edu" emails go into another.

From the main toolbar choose: Tools > Filters

Set your rules, set the action, and then choose a new or existing
mailbox.

When you are finished, select all the messages in your inbox, and,
from the main toolbar, choose: Special > Filter Messages